Process of modifying the mechanical properties of magnesium alloys



July 20, 1937. A. J. MURPHY 2,087,252- PROCESS OF MODIFYING THE MECHANtCAL PROPERTIES OF MAGNES I UM ALLOYS Filed Dec. 5, 1955 v I A I 436 I I I 300 BY m Patented July 20, 1937 I UNITED. STATES PROCESS OF MODIFYING TH E MECHANICAL PROPERTIES OF MAGNESIUM ALLOYS Alfred John Murphy, Petts Wood, England, as-

signor, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware Application December 3, 1935, Serial No. 52,655 In Germany December 15, 1934 5 Claims.

The present invention relates to improvements in the heat treatment of magnesium alloys and is particularly applicable to alloys or workpieces which are capable of being improved by a heat 5 treatment resulting in so called precipitation hardening. The invention has been mainly developed in connectionwith magnesium base alloys containing between about 3 and about 12 percent of aluminium, but its application is by no means limited thereto.

In the heat treatment of alloys with the object of causing precipitation hardening thereof it is customary to subject them in the first place to a heat treatment within a temperature range of increasing solubility of the alloying components capable of forming a solid solution (homogenizing stage) and thereupon either quenching the alloy and then annealing it"within a lowerrange of temperature, or else subjecting it to an artifically retarded cooling from the first mentioned temperature range (annealing stage).

Theoretically, the upper limit for the temperature range to be employed in the homogenizing stage would be constituted by the solidus point of the alloys; and since, on the other hand, the rate of diffusion of the alloying components increases considerably as the temperature rises, the application of the heat treatment at temperatures as close as possible below the solidus point, is indicated for the purpose of shortening the treatment,

It has, however, beenobserved tgla1 t in certain cases, and particularly with ma si u base alloys containing between about 3 and about 12 percent of aluminium the application of such high temperatures leads to partial fusion of the alloys, and thus also to distortion of the workpieces sub- Jected to the heat treatment.

It has now been ascertained that this theoretically unexpected behaviour of the alloys is attributable to the circumstance that, in cooling, they do not solidify in the state of equilibrium apparent from the phase diagram. Owing to their low rate of diffusion, the primary crystals, high in magnesium, which form in cooling, fail to take up into solid solution such amounts of the alloy components, for example aluminium, as correspond to the limit of saturation in the solid condition; consequently, larger or' smaller amounts of said components separate out in the form of an inter-crystalline network, the melting point of which seems to coincide with that of the eutectic occurring in the case of higher propor- 55 tions of foreign metal. For this reason, the only temperatures that could be employed in the heat treatment of these alloys had to be lower than said eutectic temperature and they had to be maintained for correspondingly long periods in order to produce the desired homogenizing effect. Since these periods extend over several days, the

long duration of the heat treatment is founder:-

ceedingly troublesome in practice.

It has now been observed that, even after a relatively short heat treatment at temperatures close below the eutectic temperature, the major portion of the readily fusible intercrystalline network disappears, insofar as a substantial proportion thereof enters, in the form of a solid solution, into the initially still unsaturated primary magnesium crystals. Once this condition has been attained, however, it seems that the temperature can then be raised up to and beyond the eutectic temperature, without any danger of partial fusion of the workpiece, and its undesir-.

able consequence,.by reason "of the presence of readily fusible structural components. Of course,

this must not exceed the solidus temperature corresponding, on the basis of the phase diagram, to the composition of the alloy.

The accompanying drawing represents in an entirely diagrammatic manner,.by way of example, the high magnesium side. of the phase diagram of magnesium aluminium alloys. l'.'he continuous lines A, B, C correspond to the theoretical phase limits of the alloys in a state of equilibrium, whereas the broken lines D, E, F represent the conditions actually observed" as established when the alloys are cooled from the liquidus state in the ordinary way.

According to the invention, the homogenizing stage of the heat treatment is divided into two sub-stages the first of which is of comparatively short duration and is carried out at temperatures below the eutectic temperature, but still within the temperature range of increased solubility of the alloying metal causing precipitation hardening, -whilst, in the second stage, higher temperatures, preferably exceeding the eutectic temperature, are'einployed.

The eflectof the first stage accordingly is that the main portion of the components, high in. foreign metal, located along the boundaries of the grains, is caused to disappear by difiusion in the primary crystals. With reference to the drawing, thisimplies that the broken lines D, E, F are displaced towards the left, so that they gradually draw nearer to the unbroken lines A, B, C and finally coincide with these latter.

From this, however, it follows that the temperature at which even merely a ,partial fusion of the components of the alloy occurs, is raised owing to the disappearance of these readily fusible components, and that it will finally coincide with the continuous, solidus line in the phase diagram. It follows therefrom that once this stage is attained temperatures equal to or above the-eutectlc temperature may be employed in the homogenizing heat treatment and that the homogenization can be completed in a correspondingly shorter time.

According to the method of treatment hitherto customary, a heat treatment at 430 C., extending over about 72 hours, was considered necessary for homogenizing magnesium aluminium alloys, whereas according to the present invention, in which a pro-treatment between about 420 and about 430 C. takes place during e. g. 5 to 9 hours, the same effect is already obtained by a secondary treatment of about 16 to 8 hours at about 435 and 440 C., so that the total duration of the heat treatment is only about 21 to 17 hours. A

The heat treatment at temperatures above the eutectic may immediately succeed the pre-treatment. Alternatively, the workpiece mayv be quenched after the pre-treatment, and be subjected later to the secondary treatment at the higher temperature.

Further experiments have shown that the temperature of the pre-treatment may also be gradually increased as the treatment progresses, in such a way as to correspond to the gradual approximation of the actual solidus line to its theoretical course (in accordance with the gradual approximation of the lines D, E, F to the lines A, B, C in the phase diagram). In this manner, a gradual transition from the pro-treatment stage to the secondary stage is attained.

Thus it is possible to carry out the heat treatment according to the invention with magnesium base alloys containing more than 3 percent of aluminium also by starting with a temperature of, say, 420 C. and gradually increasing the temperature to, say, 440C, the rise in temperature being substantially evenly spread over a period of from about 16 to 21 hours.

In cases in which the aluminiumcontent of the alloy is only moderate, e. g. does not exceed about 6 percent of the'total, the periods stated for the heat treatment may even be further shortened since the dissolution of the intercrystalline network in the primary crystals rich in magnesium is then more speedily attained.

Example A magnesium alloy, containing 8% of alu the temperature being then raised to 435 C.

to; 50ffurther improving: its mechanical'propand maintained thereat for 16 hours. After quenching, the alloy had a tensile strength of 20kg. per sq. mm. and an elongation of 5% (measured length, 50 mm.). Following this heat treatment, @the alloy-{may be annealed for 16 ;hqurs at 150. C., in known manner, for the put-- In addition to the foreign metals, such as aiuto 8 hours.

,minium, zinc, and the like, on whose presence the precipitation hardening effect is dependent, other elements, such as silicon, manganese and the like, which are by themselves incapable of causing precipitation hardening in magnesium,

may be added in small amounts (up to about 2%).

I claim:

1. A process of heat-treating magnesium base alloys susceptible to precipitation hardening, and containing not more than one constituent in quantities suflicient to allow solution thereof in the magnesium base crystals, to an increased extent, with rising temperatures, said process involving subjecting the alloys to a heat treatment at temperatures below the point of complete solidification of the alloy but above the point at which segregation of a constituent from the solid phase occurs, in order to cause homogenization of the crystalline texture which process comprises first subjecting the alloy to heat treatment below the eutectic temperature until the bulk of the readily fusible intercrystalline network has been absorbed by-the primary crystals high in magnesium and thereafter maintaining a temperature corresponding at least to the eu-- v tectic temperature so as to complete homogenization of the alloy.

2. A process of heat-treating magnesium base alloys containing between about 3 and about 12 percent of aluminium which comprises first subjecting the-alloy to heat treatment below about 435 C. until the bulk of the readily fusible intercrystalline network has been absorbed by the primary crystals high in magnesium and thereafter maintaining a temperature corresponding at least to about 435 C. so as to complete homogenization of the alloy.

3. A process of heat-treating magnesium base alloys containing between about 3 and about 12 percent of aluminium which comprises first subjecting the alloy to heat treatment at between about 420 C. and about 430 C. for about 5 to 9 hours and thereafter to a heat treatment be-.

tween about 435 and about 440 C. for about 16 4. A process of heat-treating magnesium base alloys susceptible to precipitation hardening, and containing not more than one constituent in quantities sufficient to allow solution thereof in the magnesium base crystals, to an increased extent, with rising temperatures, said process involving subjecting the alloys to a heat treatment at temperatures below the point of complete solidification of the alloy but above the point at which segregation of a constituent from the solid phase occurs, in order to cause homogenization of the crystalline texture which process comprises subjecting the alloy to temperatures within a range moderately below and at least equal to the eutectic temperature in such a manner that the temperature is gradually increased in correspondence with the progressive absorption of the readily fusible intercrystalline network into the primary crystals high in magnesium.

5. A process-of heat-treating magnesium base alloys containing between about 3 and about 12 percent of aluminium which comprises subjecting the alloy to increasing temperatures between about 420 and about 440 C., the rise in temperature being substantially evenly spread over a period of from about 16 to about 21 hours.

ALFRED JOHN MURRHY. 

